Hydrogen sulfide (H2S) is produced in many mammalian tissues and has been detected in micromolar amounts in blood and brain tissue. Although its potential to participate in cell signaling is clear, this biological role is not well understood. H2S, analogous to nitric oxide (NO), is produced by amino acid metabolism, readily diffuses through tissue, and is rapidly oxidized. H2S is a competent nucleophile and reductant, capable of post-translational protein modification such as ligand displacement from heme iron and dithiol reduction. In addition, H2S under physiological conditions can readily react stoichiometrically with S-nitrosothiols (RSNO) to release NO. The dynamic processes of H2S production and consumption that control cellular H2S levels respond to cellular redox status. Activity of cystathionine beta synthase (CBS) increases under oxidative stress to catalyze both H2S production and homocysteine (Hcy) breakdown. Hey, is linked to the development of atherosclerosis and neurodegeneration, and impairs NO-mediated vasorelaxation. The vascular dysfunction in hyperhomocysteinemia may be a result of decreased H2S levels. H2S is a potent vascular signal that can mediate vasoconstriction or vasorelaxation depending on O2 level and tissue. In the rat aorta, H2S concentrations that mediate rapid constriction at one O2 level will cause rapid relaxation at lower O2 levels. These results and others indicate that H2S vasoactive mechanisms include both NO-independent and NO-dependent pathways such as RSNO metabolism. The sepsis model may prove ideal for the experimental manipulation of these phenomena. Elevated vascular NO and RSNO levels during the development of sepsis contribute to hypotensive shock. H2S metabolism of RSNO under these conditions would lead to increased bioavailability of NO and may exacerbate loss of vessel tone. Because H2S, like NO, is rapidly oxidized, a novel polarographic sulfide sensor (PSS) invented in my laboratory has been a major methodological tool used to define H2S effects on vascular function. We propose to test the central hypothesis that, in the vascular, H2S is a key O2-dependent regulator of vascular function under physiological and pathological conditions. The components of this hypothesis will be explored in the following specific aims. AIM 1. Determine the homeostatic mechanisms that directly regulate H2S production and consumption with emphasis on conditions that can perturb cellular redox status AIM 2. Determine the mechanisms of O2-dependent H2S control of vessel tension under normal and inflammatory disease conditions.